System for controlling the dynamic range of electric musical instruments

ABSTRACT

A system is provided for use with an electrical musical instrument for conditioning electrical signals generated by said instrument. Said system comprises an amplifier circuit having input means for responding to said signals and output means for providing a conditioned output signal, and at least one compressing means operatively connected to said input means for responding to the amplitude of said electrical signals to provide a compressed signal to said amplifier circuit that is variable as a nonlinear function of said amplitude.

FIELD OF THE INVENTION

The present invention relates in general to dynamic range control circuits for use with electric musical instruments. More particularly, the present invention relates to a circuit for controlling the dynamic range of instrument output, while preserving tonal qualities of the instrument.

BACKGROUND OF THE INVENTION

Electronic stringed musical instruments such as guitar and bass often have a wide dynamic range, typically greater than 60 decibels. Such widely varying signals when passed through an amplifier circuit can cause problems, particularly when low voltage or battery operated electronic circuits are used to process the signals. In order to maintain high average signal levels through these circuits, dynamic range control is required.

Without some form of dynamic range control for electric music instrument amplifiers, typical active circuit components such as operational amplifiers and transistors can clip signal peaks and cause undesirable distortion. Distortion created by this type of clipping typically contains high levels of odd order harmonics (i.e., 3rd,5th, 7th . . . ,), which are considered by many to be harsh sounding and generally unacceptable if excessive levels are allowed.

One known method of dynamic range control is to use simple voltage dividing circuits or variable resistors to adjust incoming signals so that the maximum levels expected do not exceed certain predetermined limits. These circuits are usually adjusted only once. However, much of the available headroom is only used during signal transients. This results in low average signal levels and overall signal to noise ratios that are less than desirable.

Another method is to limit peak signal excursions by using diodes or transistors placed in the circuit in such a way that when signal peaks reach a certain level signals are limited from going any higher. This limiting action keeps signal peaks away from amplifier nonlinearities and clipping levels. However, such limiting action can be abrupt and create high levels of odd order harmonic components, and an acceptable level of dynamic range control without excessive distortion may be difficult to achieve in practice.

Automatic gain control (AGC) circuits are also common in the art for use with electric musical instrument amplifiers. Such AGC circuits use variable gain and level sensing circuits that set circuit gain according to signal level. Such circuits can be effective and solve the problem of low average signal levels of the previous methods, but because much of the uniqueness of different musical instrument sounds is contained within the transients and peaks and valleys of the signal produced by these instruments, the individuality of instrument and player tends to be eliminated by using AGC devices, thereby tending to make many instruments sound somewhat the same. Furthermore, the basic AGC circuit building blocks can be relatively complex and costly. AGC circuits also usually have fixed attach and release times which further restricts the individual tonal quality of the instrument.

It is also known to use nonlinear circuits to introduce a controlled amount of distortion for obtaining a desired harmonic content as discussed in two publications by Richard A. Shaeffer entitled "Electronic Musical Tone Production by Nonlinear Waveshaping" in Volume 18, Number 4 of the Journal Of The Audio Engineering Society at pp. 413-416 (1970) and "Production of Harmonics and Distortion in p-n Junctions" in Volume 19, Number 9 of the Journal Of The Audio Engineering Society at pp. 759-768 (1971).

Other examples of peak limiting, clamping, and compression circuits are generally shown in U.S. Pat. Nos. 3,509,373; 3,986,049; 3,548,323; 4,119,922; and 4,349,788.

SUMMARY OF THE INVENTION

In accordance with the preferred embodiment, a circuit for use with an electric musical instrument is provided. A compressor circuit is included with an operational amplifier having compressing means both on the input side and in the feedback loop. Each compressing means includes two oppositely connected back to back diodes operated in the forward conducting region of the diode characteristics, but unlike prior art devices, operated primarily in the nonlinear region of the diode forward characteristic. In this manner, a controlled amount of distortion is introduced to produce a more pleasing sound, which is therefore believed to preferentially emphasize the even order harmonics. In this fashion, a more pleasing response is observed that preserves tonal characteristics of the instrument being played, while introducing a controlled degree of distortion.

In a further feature of the invention, the output of the compressor circuit can be passed through a series combination of a narrow bandpass filter, a high gain limiter, and a low pass filter for creating a relatively highly compressed, harmonically complex, distorted sound most frequently used in "hard rock" music.

The present invention provides a circuit that requires relatively few components and is particularly well suited for use with low voltage or battery operated devices such as headphone amplifiers and signal processing devices for electric guitar and bass.

The present invention also provides an effective method of dynamic range control for use with stringed electric musical instruments that tends to preserve the individual tonal character of the instrument.

The present invention further provides a circuit that consumes relatively low power.

The present invention also provides a circuit that can be used in such applications as battery operated devices, for example with musical instrument signal processing devices and headphone amplifiers.

Further objects and advantages of the present invention will become apparent from the following description of the drawings and the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of the preferred embodiment of the present invention;

FIG. 2 is a schematic circuit diagram of the compressor circuit of FIG. 1;

FIG. 2A is a curve explaining the operation of the compressor circuit of FIG. 2.

FIG. 3 is a schematic circuit diagram of the bandpass, high gain limiter, and low pass circuits of FIG. 1;

FIG. 4 is a schematic circuit diagram of the high boost circuit and power amplifier circuit of FIG. 1; and

FIG. 5 is a schematic circuit diagram of the power circuitry for the circuit of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference to FIG. 1, an electric guitar 2 is shown connected by lead 4 to the circuit 10 of the present invention, which preferably comprises the compressor circuit 12 of the present invention. If switch 76B, which is further described below, is open as shown in FIG. 1, then the circuit 10 of the present invention also includes bandpass filter circuit 14, high gain limiter circuit 16, and low pass filter circuit 18. If switch 76B is closed, then filter 14, limiter 16, and filter 18 are effectively removed from circuit 10. High boost circuit 20 and power amplifier circuit 154 complete circuit 10. Each of these individual circuits is discussed in further detail below. The output of circuit 10 is connected via lead 6 to a set of headphones 8. Circuit 10 is preferably housed within a rigid body for attaching directly to the body 3 of guitar 2 as described in my copending, simultaneously filed, patent application entitled "Portable Musical Instrument Amplifier", which is hereby incorporated by reference.

Turning now to FIG. 2, numeral 22 indicates the input terminal to the circuit 10 of the present invention. Terminal 22 receives its electrical signal input from a routine transducer 5, such as a well known electric guitar pickup having at least one coil 7 for generating an electrical signal having an amplitude that typically ranges from 0-1.0 volt RMS, through a connector such as jack 36. Transducer 5 also includes routine and well known control circuitry indicated generally at 9, the details of which form no part of the present invention. Terminal 22 is connected through capacitor 38 to junction 40, which is connected to a parallel combination of capacitor 42 and resistor 44. This parallel combination is connected to terminal 46, which is the non-inverting input of operational amplifier 48. Operational amplifier 48 may be any suitable amplifier circuit, such as for example LM 358 manufactured by National Semiconductor. Such operational amplifiers typically have an input port or noninverting input indicated by (+), a feedback port or inverting input indicated by (-), and an output port or terminal. It is understood that the operational amplifiers shown in FIGS. 2 through 4 are routinely connected to standard voltage supplies such as for example V_(ref) and V_(s) as indicated in FIGS. 2 through 5, even though the actual connections are not shown. The power supply will be discussed in further detail below with respect to FIG. 5.

The output terminal 50 of operational amplifier 48 is connected to junction 24. Terminal 50 is also connected through a feedback loop comprising the parallel combination of capacitor 52, oppositely connected semiconductor diodes 54 and 56 in parallel, and series resistors 58 and 60 to the inverting input 62 of operational amplifier 48. Terminal 62 is connected to ground through resistor 64 and capacitor 66. Output 50 of operational amplifier 48 is connected to ground through resistor 68, and terminal 46 is connected to V_(ref) through the parallel combination of resistor 70 and connected parallel semiconductor diodes 72 and 74.

An important feature of the compressor circuit 12 of the present invention is that the oppositely connected diode pair 72 and 74 connected to non-inverting input 46 and the oppositely connected diode pair 54 and 56 connected in the feedback loop to the inverting input 62 are arranged to operate primarily in the nonlinear region of each diode's forward characteristics, which is for silicon between approximately 0 and 0.7 volt.

The forward biased current voltage characteristics of p-n junctions, such as for example silicon diodes, are well known as illustrated by plot 63 of instantaneous current i versus voltage v in FIG. 2A. As forward voltage v is increased from 0 to some positive value as indicated along abscissa 65, forward resistance, which is represented by the inverse slope of curve 67, decreases until the diodes' forward voltage drop is approximately 0.60-0.70 volt, as shown by dotted line 69. At and beyond this dotted line 69 but less than breakdown voltage, the diode is considered "on" and the forward resistance is essentially constant and less than about 1.0 Ω as represented by the essentially straight portion 71 of curve 67. Most prior art limiting circuits for musical instruments are designed to operate in this "on" region.

However, the circuit of the present invention is designed to operate primarily in the nonlinear region 73 of the diodes' forward characteristic between about 0 and 0.70 volts. In this region the diode acts as a voltage variable resistor, as represented by the varying slope of curve 67 in region 73. Because the diodes 72 and 74 are set up as the shunt elements in a voltage divider circuit comprising resistors 44 and 70 and capacitor 42, the signal wave form from the coil 7 is modified nonlinearly in that as the amplitude of the signal at terminal 22 increases, the shunt resistance seen at terminal 46 decreases. When the diode shunt resistance approaches that of resistor 70, input signal level begins to be controlled by diode forward resistance with amplifier output being reduced nonlinearly with increased signal amplitude. High peak excursions of the input signal will be limited to ±0.70 volt in a manner similar to that of limiting circuits. In this way the dynamic range of the input signal is reduced.

Input capacitor 38 sets the low frequency response of the circuit and AC couples the input signal from coil 7 to the remainder of the circuit. Resistors 44 and 70 in combination with capacitor 42 are important for conditioning the desired portion of the input signal to operate primarily in the nonlinear region 73 of the shunt diodes 72 and 74 forward characteristic. The values of resistor 44 and resistor 70 can be varied from instrument to instrument to get the input signal to swing through the proper voltage range. For an electric guitar, the preferred voltage divider ratio of the resistance value of resistor 70 to the sum of the resistance values of resistors 70 and 44 is about 0.25, which is given by setting resistor 44 to 1MΩ and resistor 70 to 300KΩ. Other ratios, such as in the range of about 0.1 to 1.0 can be used depending on the effect desired and instrument used.

Diode 72 operates during positive excursions of the input signal while diode 74 operates during the negative cycles. For low level signals the forward resistance of diodes 72 and 74 is high relative to resistor 70. Therefore the signal passes through to operational amplifier 48 relatively unaffected by the shunt action of the diodes. As the signal level is increased, the diodes' shunt resistance begins to decrease, thereby reducing circuit gain and compressing signal dynamic range. Capacitor 42 is used for high frequency preemphasis, which may or may not be desirable depending on the source instrument and the tonal response desired. The diode action is significant in maintaining the tonal character of the specific musical instrument being used.

Accordingly, capacitor 42, resistors 44 and 70, and diode pair 72 and 74 act as a first compressor means for compressing the signal input at 22 by compressing signal amplitude changes as the signal is applied to terminal 46. The compressed signal at 46 is therefore a nonlinear function of the input at 22 and is coupled to the positive or non-inverting input of operational amplifier 48. It is also understood that the first compressor means could be repeated in series to provide successive stages of compression.

The basic gain of the amplifier 48 is set by resistors 58, 60, and 64. Gain can be manually adjusted by manually operable variable resistor 60 to provide amplifier input level control so that the feedback signal again operates primarily in the nonlinear region of feedback diodes 54 and 56. In practice resistor 70 or 44 can also be made user adjustable so that optimum setting can be made for the particular instrument/player combination being used. Because the resistance of diodes 54 and 56 varies as a nonlinear function of the output signal, capacitor 52, diodes 54 and 56, and resistors 58 and 60 have a variable feedback impedance providing a second compressor means for compressing the output signal even further by reducing amplifier gain in response to increased amplifier output at 24.

An important feature of the present amplifier is that as amplifier input signals at 46 are increased, amplifier gain is reduced by the action of diodes 54, 56 and the feedback loop to approach a minimum value of unity for the noninverting amplifier configuration used. Unlike the first compressor means, which limits peak excursions to 0.7 volts, the second compressor means including capacitor 52 and diodes 54 and 56 as part of the feedback loop for operational amplifier 48 reduces the gain, but to no less than unity. This feature further helps to retain some of the tonal individuality of the instrument, because signal transients compressed by the first compressor means are not clipped or "hard" limited by diodes 54 or 56, which would create unwanted distortion, but instead are allowed to pass at no less than unity gain while low level signals little affected by first compressor means are amplified relative to these peaks to further reduce overall signal dynamic range.

Capacitor 52 is used to reduce high frequency noise and to attenuate any higher order harmonics present.

The resultant wave form at the output 24 of amplifier 48 is a compressed version of the input at 22, with low level signals being passed through with relatively little alteration to tonal quality and higher levels being increasingly attenuated with additional subtle harmonic content.

By operating primarily in the nonlinear region, compressor circuit 12 compresses the dynamic range of a widely varying AC waveform and has a resultant distorted wave form having improved and more pleasing sound, believed to be caused by increased even order harmonic content. This is of particular advantage to electronic stringed musical instruments because the addition of even order (2nd, 4th, etc.) harmonics to the signal is generally believed to be either unnoticed or considered a welcome addition as long as the fundamental frequency domain characteristics of the original signal are left intact.

Depending upon the setting of switch 76 as described in further detail below, the signal output from compressor 12 at terminal 24 can be applied to the bandpass filter 14, high gain limiter 16, and low pass filter lB as described hereinafter.

FIG. 3 shows the bandpass filter circuit 14 of FIG. 1 between junctions 24 and 26 the high gain limiter circuit 16 of FIG. 1 between junctions 26 and 28; and the low pass filter 18 of FIG. 1 between junctions 28 and 30. Junctions 24, 26, 28, and 30 of FIG. 3 of course correspond to like junction numbers in FIGS. 1, 2, and 4 and so on.

With reference now to FIG. 3, switch 76 is shown in two parts as switches 76A and 76B ganged together as indicated by dotted line 78. The setting of switch 76 establishes whether circuit 10 of FIG. 1 is in a state corresponding to switch setting 80, setting 82, or setting 84, which corresponds to the settings "clean" or little distortion; "edge" or subtle distortion, and "heavy metal" or substantial distortion, respectively as hereinafter described.

If switch 76 is in the "clean" position or position 80 as it happens to be in FIG. 3, the signal output from compressor 12 is fed directly through resistor 86 to the high boost and power amplifier circuits of FIG. 4, thereby bypassing bandpass filter 14, high gain limiter 16, and low pass filter 18.

If switch 76 is in either position 82 or 84, the signal at 24 passes through bandpass filter 14, high gain limiter 16, and low pass filter 18 before reaching the high boost and power amplifier circuits of FIG. 4.

With reference now to FIG. 3, bandpass filter 14 is for preemphasizing a selected portion of the instrument's output frequency spectrum. This is a common bandpass circuit known as a multiple feedback bandpass filter. Filter 14 includes resistor 88 for feeding the signal through capacitor 90 to the inverting input 91 of operational amplifier 92, the output of which is connected to switch 76A and junction 26 when switch 76 is in position 84. Operational amplifier 92 has a feedback loop with resistor 98 connecting the output of operational amplifier 92 to inverting input 91. Capacitor 100 also feeds a portion of the output of operational amplifier 92 back through capacitor 90 to the inverting input 91. The non-inverting input 102 of operational amplifier 92 is connected to V_(ref), and the output is also grounded through resistor 104.

The bandpass filter 14 is designed to have a resonant frequency in the range of about 300 Hz to about 3,000 Hz, preferably about 985 Hertz, and a Q in the range of 0.5 to 3.0, preferably about 1.6 for use with electric guitar. Filter 14 therefore has an amplitude response such that the desired frequency preemphasis is applied to the high gain limiting stage to achieve the desired broad-band harmonic content and tonal character. The bandpass circuit 14 could be set up with other response shapes depending on the particular effect or sound desired. In practice filter 14 could be made variable to accommodate different guitars or for adjustment to an individual musician's taste.

After passing through bandpass filter 14, the signal now passes through a high gain limiting stage 16 to produce a relatively high distorted, long sustain signal at 28. Capacitor 106 couples the limiting circuit 16 to junction 26. Resistor 108 is connected to the inverting input 109 of operational amplifier 111. The parallel combination of capacitor 110, back-to-back oppositely connected diodes 112 and 114, and resistor 116, comprise a feedback loop from operational amplifier output 113 back to inverting input 109. Output 113 is also connected to ground through resistor 115. Non-inverting input 117 is connected to V_(ref). Resistors 108 and 116, capacitors 106 and 110, diodes 112 and 114, and operational amplifier 111 comprise a relatively high gain limiting stage, with diodes 112 and 114 being operated substantially in the linear or "on" region of the diode forward characteristics.

Amplifier 111 is preferably designed to have a gain of about 212 volts/volt for use in the substantial distortion mode setting 84 of switch 76, as opposed to about 9.6 volts/volt in the edge mode setting 82 of switch 76. Amplifier 111 has a low frequency corner of about 338 Hz in the distortion mode; 691 Hz in the edge mode; and a high frequency rolloff beginning at about 723 Hz, so that desired distortion level and harmonic content are achieved with proper signal sustain.

The high gain limiter 16 works in conjunction with the pass band filter 14 to create a specific harmonic content and tonal character in the following way. Signals in the band pass of filter 14 are amplified by limiter 16 to a high enough level so that diodes 114 and 112 are operating in the linear or full "on" region over a wide range of input amplitudes (212 v/v amplifies 0.003 volts peak or 0.006 volts peak-to-peak up to limit). Signals in this frequency region are substantially distorted with peak to peak amplitudes limited to approximately ± 0.7 V peak (1.4V peak-to-peak). This limiting action introduces substantial harmonic distortion of all orders. Therefore, in the pass band, signals are "hard limited" producing a significant content of odd as well as even order harmonics.

By contrast, signal frequencies above and below the pass band of filter 14 are amplified by limiter 16, but because the amplitudes of these signals become smaller and smaller as the frequencies are further removed from the pass band signals, diodes 112 and 114 are driven thereby to a lesser degree. Therefore, depending on the specific frequency, diodes 112 and 114 can be operated in all regions of the diodes' forward characteristic. By contrast to pass band signals, which are substantially distorted as explained above, harmonic distortion of frequencies above and below pass band signals becomes less and less the further from the pass band frequencies the signal becomes. Out of band signals are distorted to an increasingly less degree operating more and more in the nonlinear region of the diodes' characteristic. These output signals display gently rounded as opposed to sharp corners. Sharp corners are believed to indicate the presence of odd order harmonics.

After passing through stage 16, the signal is fed through low pass filter 18 to reduce high frequency harmonic content and noise levels. Output 113 of operational amplifier 111 is connected through the voltage dividing pair of resistors 118 and 120 to the non-inverting input 122 of operational amplifier 124, the output 126 of which is connected, for switch position 84, through resistor 138 of switch 76 to junction 30. Output 126 is also connected directly to the inverting input 130 of operational amplifier 124, which inverting input is also coupled by capacitor 131 to the junction 29 between resistors 118 and 120. Non-inverting input 122 is connected to V_(ref) through capacitor 132. Output 126 is also grounded through resistor 134. Terminals 135 and 137 of operational amplifier 124 are connected to V_(s) and ground respectively as shown.

Resistors 118 and 120, capacitors 131 and 132, and operational amplifier 124 comprise a second order Chebechev 1dB ripple low pass filter with a corner frequency in the range of about 200-4,000 Hz, preferably 3,000 Hz, which filters out the higher harmonics which are generally responsible for harsh or gritty sound of signals emitting from amplifier 111. This filter is effective in reducing the substantial higher order harmonic content created by limiter 16 in combination with the preemphasis action of filter 14.

When switch 76 is set in position 82, the capacitor 94 and resistor 96 are switched into the circuit 14 to provide decreased gain for limiter 16. Accordingly, in this switch position a small resistor 128 replaces resistor 138 to compensate for the decreased signal level. This changes the signal so that the distortion introduced by limiter 16 is much less, primarily distorting signal peaks giving an "edge" to the sound, with distorted signal peaks displaying rounded as opposed to sharp corners.

Turning now to FIG. 4, the high boost circuit 20 is shown between junctions 30 and 31. Circuit 20 is in essence a treble control for adjusting the degree of "bite" by increasing signal high frequency content. Capacitor 140 is connected to ground through logarithmic potentiometer 142 for volume control, which is in turn connected to potentiometer 144 for "bite" control by varying the amount of high frequency boost. Resistor 146 couples potentiometer 144 through capacitor 148 to junction 31, which is also connected to ground through resistor 150. The signal at 31 passes through power amplifier 154 to output 153 and then through capacitor 156 to a connector such as jack 34, preferably for connection to headphones 8. Junction 31 is connected to the non- inverting input 158 of power amplifier 154, while the inverting input 160 is grounded. Terminals 162 and 164 of power amplifier 154 are connected to V_(s) and ground respectively. Output 153 is also connected to ground through resistor 166 and capacitor 168.

Connector 32 is for an auxiliary input through resistor 152 for allowing an auxiliary signal to be superposed, such as for example from a tape player or other source with which the musician wishes to play along. This auxiliary signal at 32 sets up a voltage drop across summing resistor 150, which permits the signal from high boost circuit 20 to be summed therewith.

FIG. 5 shows the preferred power supply for the circuit of the present invention. Battery 170 is preferably a 9 volt alkaline dry cell connected through a standard connector such as battery snap connector 172 to the circuit of the present invention. Negative terminal 174 is connectable through on-off power switch 176 to ground to power-up the circuit. Positive terminal 176 is connected to terminal 178 for providing V_(s) of approximately +9 volts for the various circuit elements as indicated above. Battery 170 is also connected through resistors 180 and 182 to terminal 184 for providing V_(ref) of approximately +1.7 volts, which is coupled to ground by capacitor 186 for reducing noise and other undesirable stray signals. LED 188 provides a visual indication of when the circuit is "on". Resister 182 and capacitor 186 comprise a low pass filter to provide a stable V_(ref) for circuit operation.

The following listing provides the values for the circuit of the preferred embodiment:

    ______________________________________                                         Resistors (all resistors are 1/8 W, 5%, carbon film)                           44           1 MΩ                                                        58           10KΩ                                                        60           50KΩ  linear potentiometer                                  64           6.8KΩ                                                       68           6.8KΩ                                                       70           300KΩ                                                       88           15KΩ                                                        96           100KΩ                                                       98           160KΩ                                                       104          6.8KΩ                                                       108          4.7KΩ                                                       115          6.8KΩ                                                       116          1 MΩ                                                        118          120KΩ                                                       120          120KΩ                                                       128          2.4KΩ                                                       134          6.8KΩ                                                       138          15KΩ                                                        142          10KΩ  logarithmic potentiometer                             144          50KΩ  linear potentiometer                                  146          10KΩ                                                        150          10KΩ                                                        152          160KΩ                                                       166          10 Ω                                                        180          3.0KΩ                                                       182          3.0KΩ                                                       Capacitors                                                                     38           2,200 picofarads                                                  42           330 picofarads                                                    52           1,000 picofarads                                                  66           0.1 microfarads                                                   90           3,300 picofarads                                                  94           2,200 picofarads                                                  100          3,300 picofarads                                                  106          0.10 microfarads                                                  110          220 picofarads                                                    131          1,000 picofarads                                                  132          270 picofarads                                                    140          0.22 microfarads                                                  148          3,300 picofarads                                                  156          100 microfarads, 10 V                                             168          0.10 microfarads                                                  177          4.7 microfarads, 10 V                                             186          4.7 microfarads, 10 V                                             Diodes                                                                         54           1N4148                                                            56           1N4148                                                            72           1N4148                                                            74           1N4148                                                            112          1N4148                                                            114          1N4148                                                            188          HLMP 1700                                                         Amplifiers                                                                      48           LM 358                                                                                       Dual                                               92           LM 358                                                             111          LM 358                                                                                       Dual                                               124          LM 358                                                            154          LM 386                                                            ______________________________________                                    

The operational amplifiers are standard chips connected to standard supply voltages V_(ref) and V_(s) as indicated in the drawings. It is understood that any of the operational amplifiers could be replaced by routine and suitable amplifier circuitry, such as for example a transistor-based

While specific values of voltages and components have been described, it is understood that such values are only exemplary and any equivalent values or set of values may be used which attain the described results.

It should be understood that various changes and modifications to the preferred embodiments described above will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present invention, and it is therefore intended that such changes and modifications be covered by the following claims. 

What is claimed is:
 1. A signal control circuit for an electrical musical instrument comprising:(a) an amplifier circuit having a noninverting input port for receiving an electrical musical instrument input signal to be controlled, said input signal having an amplitude range varying between minimum and maximum values, an output port for supplying a controlled output signal, and an inverting input port for receiving a feedback signal from said output port for controlling amplifier gain; (b) input compressing circuit means operatively coupled to said noninverting input port for compressing the amplitude of the input signal applied to said input port to a compressed input signal having an amplitude no greater than about a predetermined maximum level, said input compressing circuit means also for converting said compressed signal to a substantially nonlinear function of said input signal to preferentially emphasize desired harmonic content of said compressed signal; and (c) feedback circuit means operatively coupling said output port to said inverting input port for feeding back a portion of said output signal as a feedback signal, said feedback circuit means including feedback signal compressing means for compressing said feedback signal to a compressed feedback signal having an amplitude no greater than a preselected maximum value, said feedback circuit means also for reducing the gain of said amplifier as a nonlinear function of said output signal as said output signal increases,whereby said output signal has preferentially emphasized harmonic content and has an amplitude compressed within a selected range relatively independently of the range of said input signal.
 2. The signal controlling circuit of claim 1, wherein said amplifier circuit is an operational amplifier, said input compressing circuit means includes first and second resistors in series for dividing input signal voltage, the first of said resistors being connected between the noninverting input and a reference voltage source, and a first pair of back-to-back oppositely connected semiconductor diode means, said pair being connected in parallel with said first resistor, said first and second resistors having resistance values chosen to operate said diode pair primarily in the nonlinear region of the diode forward characteristics.
 3. The signal controlling circuit of claim 2, where the ratio of the resistance of the second resistor to the sum of the resistances of said second plus said first resistors is approximately in the range of 0.1 to 1.0.
 4. The signal controlling circuit of claim 3, wherein said ratio is about 0.25.
 5. The signal controlling circuit of claim 4, wherein said feedback signal compressing means includes a second pair of back-to-back oppositely connected semiconductor diode means in parallel with a third resistor means.
 6. The signal controlling circuit of claim 5, wherein said third resistor mean includes manually operable variable resistor means for manually adjusting said gain.
 7. The signal controlling circuit of claim 6, wherein the minimum gain of said operational amplifier is unity.
 8. The signal controlling circuit of claim 7, further including first capacitor means in parallel with said second resistor, and second capacitor means in parallel with said second pair of diode means.
 9. The signal controlling circuit of claim 8, wherein said first resistor has a resistance value of substantially about 300 K Ω and said second resistor has a value of about 1 M Ω.
 10. The signal controlling circuit of claim 9 further comprising bandpass filter means operatively connected to said output port for further conditioning said output signal, said bandpass filter means having a resonant frequency substantially in the range of about 300 to 3,000 Hertz and a Q in the range of substantially about 0.5 to 3.0.
 11. The signal controlling circuit of claim 10, wherein said resonant frequency is about 985 Hertz and a Q of about 1.6.
 12. The signal controlling circuit of claim 11, further comprising amplitude compressing means operatively coupled in series with said bandpass filter means.
 13. The signal controlling circuit of claim 12, further comprising low pass filter means operatively coupled in series with said bandpass filter means and said amplitude compressing means.
 14. A system for use with an electrical musical instrument for conditioning electrical signals generated by said instrument, said system comprising:(a) an amplifier circuit having input means for responding to said signals and output means for providing a conditioned output signal; (b) at least one compressing means operatively connected to said input means for responding to the amplitude of said electrical signals to provide a compressed signal to said amplifier circuit that is variable as a nonlinear function of said amplitude; and (c) a feedback circuit including a second compressing means connected between said output means and said input means of said amplifier circuit, said feedback circuit having feedback impedance that varies as a nonlinear function of at least the amplitude of the output signal.
 15. The circuit of claim 13, wherein first said compressing means includes first semiconductor means operatively connected in a signal path supplying said input means.
 16. The circuit of claim 15, wherein said second compressing means includes second semiconductor means operatively connected in said feedback circuit.
 17. The circuit of claim 16, wherein said first and second semiconductor means each include at least one p-n junction.
 18. The circuit of claim 17, wherein said first and second semiconductor means each include at least one semiconductor diode.
 19. The circuit of claim 16, wherein said first and second semiconductor means each include two parallel and oppositely connected diode paths.
 20. The circuit of claim 16, wherein said first and second semiconductor means each include a pair of parallel and oppositely connected diodes for being operated in their nonlinear regions for a substantial portion of the range of the electrical signals.
 21. The circuit of claim 20 further comprising a voltage divider circuit on the input side of said amplifier circuit, and one said diode pair is operatively connected as a shunt element in said divider circuit, said oppositely connected diode pair for being operated substantially in said nonlinear region of the diode forward characteristics.
 22. The circuit of claim 21 wherein said voltage divider circuit includes at least a pair of resistors, one of said resistors being in the path coupled to said input means and the other resistor being connected in parallel with said diode pair.
 23. A dynamic range control circuit for an electric musical instrument comprising:(a) an amplifier circuit having an input port for receiving input signals, a feedback port, and an output port; (b) feedback means operatively connected between said output port and said feedback port for feeding back a portion of a signal at said output port to said feedback port, said feedback means including means for controlling a portion of said signal being fed back to reduce the amplitude of said signal at said output port, said feedback means having a signal response characteristic having at least one nonlinear region for a substantial part of said characteristic, said feedback means being operated substantially in said nonlinear region; and (c) controlling circuit means operatively coupled to said input port for reducing the amplitude of input signals applied to said input port.
 24. The dynamic range control circuit of claim 23, wherein said controlling circuit means has a signal response characteristic that is nonlinear for a substantial region of said characteristic, said controlling circuit means for being operated substantially in said nonlinear region.
 25. The circuit of claim 24, wherein said controlling means includes a pair of back to back semiconductor p-n junctions.
 26. The circuit of claim 25, further comprising a voltage dividing circuit having at least a first resistance in shunt with said p-n junctions and a second resistance coupled to ground.
 27. The circuit of claim 26, further comprising a variable resistance in series with said first resistance for adjustably controlling the gain of said amplifier circuit.
 28. The circuit of claim 27, further including a capacitor controlled in parallel with said p-n junctions.
 29. A circuit for controlling electrical signals from at least one transducer associated with an electrified musical instrument, said circuit having input and output terminals and comprising:(a) bandpass filter circuit means operatively coupled to said transducer for receiving signals therefrom and passing therethrough a relatively narrow bandwidth of said signals, said bandpass filter means having a resonant frequency substantially about 985 Hertz and a Q of about 1.6; limiting circuit means operatively connected to said bandpass filter circuit means for limiting the signals received from said filter circuit means, said limiting circuit means including at least a pair of oppositely connected semiconductor diode paths for operating substantially in the linear region of said diode forward characteristics; (c) low pass filter circuit means operatively coupled to said limiting circuit means to further condition the signals; and (d) compressor circuit means operatively coupled between said transducer and said bandpass filter circuit means, said compressor circuit means for compressing the signals applied to said input terminals to less than a predetermined constant, said compressor circuit means having at least one pair of oppositely connected parallel diode paths for operating in the nonlinear region of the diode forward characteristic.
 30. The system of claim 29, wherein said compressor circuit means includes an operational amplifier having a non-inverting input, an inverting input, and an output in the signal path between the signal input terminal and the output terminal with a feedback path containing a pair of oppositely connected parallel diode paths connecting the amplifier inverting input to the amplifier output and compressing means coupling said input terminal to said non-inverting input and having a nonlinear response characteristic for compressing the amplitude of signals applied to said non-inverting input.
 31. The system of claim 30, wherein said diodes and said compressing means each comprise a pair of oppositely connected diode paths for being operated in the nonlinear region of the diode forward characteristics.
 32. The system of claim 31, wherein each diode is operatively connected to its own respective voltage divider means being variably adjustable to adjust the tonal response of said system. 